Strong Sonars Can Make Dolphins Deaf

According to an investigation into a suspected link between naval operations and cetacean strandings (two weeks ago we wrote about 80+ whales and dolphins that beached themselves in Australia), very loud blasts of sonar can cause a dolphin to temporarily lose its hearing. "A paper published in the British journal Biology Letters on Wednesday provides the first lab-scale investigation into this idea, although its authors stress it does not provide proof that warship sonar is to blame." Read on for more details.Via Discovery News:

Marine biologists led by Aran Mooney at the University of Hawaii exposed a captive-born, trained Atlantic bottlenose dolphin to progressively louder pings of mid-frequency sonar.

The experiment took place in open water pens at the Hawaii Institute of Marine Biology and in the presence of the dolphin's trainer.

The scientists fitted a harmless suction cup to the dolphin's head, with a sensor attached that monitored the animal's brainwaves.

When the pings reached 203 decibels and were repeated, the neurological data showed the mammal had become deaf, for its brain no longer responded to sound.

The deafness, though, was only temporary and the dolphin was not hurt in the experiment, said Mooney.

Other sensors showed that the dolphin's breathing rose significantly when the sonar was turned on.

The dolphin's hearing was back after 20 minutes, typically, but this makes us wonder if even louder sonar pings could make the hearing loss permanent? Is it possible that classified military sonars are louder than what researchers think is in use?

The positive part of the study is that for dolphins to perceive sonar pings this loudly, they'd have to be pretty close to the source, and stay around for a couple of minutes. This is probably pretty rare, but sounds travels different in water than air, and it can "sometimes get trapped at the surface, in layers called thermoclines, at the top 325 feet or so."

More study is probably needed to determine if these sonars are the direct cause of the beachings, or one of many factors, or not a significative cause.